OH NH 3 + OH NH 3 + Stirring n Instantaneous formation of CS/PEO-PPO nanoparticles Figure 6 Scheme for preparing CS nanoparticles porosity of the matrix of the freeze-dried hydrogel compared to that of the air-dried hydrogel (23) The ionic interaction between the positively charged amino groups of chitosan and negatively charged counterion of tripolyphosphate (TPP, MW 4000) results in polyelectrolyte complex formation and thus allows the formation of beads in very mild conditions So, chitosan nanoparticles can be prepared by ionic gelation with the counterion TPP (24) The particle size depends on both the chitosan and TPP concentrations The minimum size (260 nm) has been obtained The PEO or PEO–PPO block polymer Synperonic® (1C1 Iberica, Spain) can be incorporated into chitosan nanoparticles by dissolving these copolymers in the chitosan solution either before or after adding the ionic cross-linker TPP (Fig 6) TEM observation reveals that the chitosan nanoparticles have a solid and consistent structure, whereas chitosan/PEO–PPO nanoparticles have a compact core, which is surrounded by a thin but fluffy coat presumably composed of amphophilic PEO–PPO copolymer (25) Here, the presence of PEO–PPO within the chitosan nanoparticles can mask the ammonium group of chitosan by a steric effect, thus hindering the attachment of the BSA (isoelectric point pH = 4.8) These hydrophilic chitosan/ethylene oxide–propylene oxide block copolymer nanoparticles are very promising matrices for administering therapeutic proteins and other macromolecules that are susceptible to interaction with chitosan (i.e., genes or oligonucleotides) (26) Mucoadhesive drug delivery systems can easily be combined with auxiliary agents, such as enzyme inhibitors Chitosan and EDAC [1-ethyl-3-(3-dimethylamino-propyl) carbodimine hydrochloride] form chitosan–EDAC conjugates (10 mL of 1% chitosan HCl, 0.1M EDAC, and an amount of EDAC that ranges from 0.454 to 7.26 g) at pH 3.0 The chitosan–EDAC conjugates offers several in gene therapy, where genes are applied to prod peutic proteins in the patient Oligonucleotides, small synthetic DNA designed to hybridize speci sequences, are used to block gene expression T these goals, the gene drugs must be administer appropriate route and be delivered into the int site of the target cells where gene expression occ Gene drugs have substantial problems as po nucleic acids, including susceptibility to degradat cleases and low permeability So, a suitable carri is the key to successful in vivo gene therapy Co that viral vectors have a number of potential li involving safety, cationic lipid polymers develope carriers can improve in vivo transfection efficien Chitosan as a natural amino polysaccharide c polyelectrolyte complex with DNA For site-spe delivery, a quaternary ammonium derivative of t chitosan with antenna galactose was synthesiz indicated that the galactose-carrying chitosan as a ligand provides cell-specific delivery of DN G2 cells The chitosan derivative binds to DNA trostatic interaction The resulting complexes re ability to interact specifically with the conjugat on the target cells and lead to receptor-mediate tosis of the complex into the cell (28) (Fig 7) Separation Membranes Chemically modified chitosan membranes have in various fields, for example, metal-ion separ separation, reverse osmosis, pervaporation s of alcohol–water mixtures, ultrafiltration of macromolecular products, and affinity precip protein isolates Pervaporation is a very useful membrane s technique for separating organic liquid mixture azeotropic mixtures and mixtures of materials close boiling points In pervaporation, the chara of permeation and separation are significantly go the solubility and diffusion of the permeates The separation mechanism of pervaporation on the solution–diffusion theory, the adsorption– desorption process of the components in the fee across the membrane from one side to the other T pervaporation properties can be improved by e the adsorption of one component in the feed to brane and/or accelerating the diffusion of one c in the feed through the membrane Receptor Osmatic pressure sink Receptor mediated endocytosis −6 −5 −4 −3 lgCAICI3 −2 −1 Figure 8 Separation properties of an isopropanol–wat through a chitosan–silk fibroin complex membrane by tion [silk fibroin content in membrane: 30% (w/w), isop feed: 85% (w/w), CAlCl3 denotes the AlCl3 concentration ter part of isopropanol–water mixture] Figure 7 Scheme of gene delivery system via receptor-mediated endocytosis Chitosan has been used to form pervaporation membranes for separating water/alcohol mixtures and shows good performance in dehydrating alcohol solutions To enhance fluxes with more free volume, chitosan membranes cross-linked with hydrophilic sulfosuccinic acid (SSA) were developed where two carboxylic acid groups and one sulfonic acid group offer ionic cross-linking with amine side groups of the chitosan molecules Because the SSA crosslinked membrane bears more binding sites for the target compound to be separated than the cross-linked chitosan membrane, the former membrane is better than the latter membranes reported earlier (29) Due to high permeability and good mechanical properties comparable to those of commercial cellulose acetate membranes, the membranes have potential application in pervaporation separation of aqueous organic mixtures (30) The pervaporation properties of isopropanol–water mixture via a chitosan–silk fibroin complex membrane are shown in Fig 8 The data imply that the flux through the complex membrane expresses ion (Al3+ ) sensitivity, so the pervaporative flux of the isopropanol–water mixture can be modulated; the high selectivity was maintained by optimizing the AlCl3 concentrations in the feed (31) A benzoylchitosan membrane was designed for separating a benzene/cyclohexane mixture, which is very important in the petrochemical industry The benzene permeation selectivity of the membrane is attributed to the smaller molecular size and higher affinity of the benzene molecules compared to those of cyclohexane (32) Anionic surfactants for example, sodium lauryl ether sulfate (SLES), can form complexes with the chitosan chain through interaction of their opposite ionic charges The complexes can survive as a skin layer (ca 15 µm) on a polyethersulfone ultrafiltration membrane Here, the ionic property of the surfactant–chitosan complex membrane preferentially promotes the permeation of polar methanol through the membrane, compared with the less relative bulky methyl-t-butylether (MTBE) (33) A formed-in-place (FIP) membrane is dynami ated by depositing polymers on the surface or a trance of the pores of macroporous substrates FIP ultrafiltration membranes can be formed on porous titanium dioxide substrate The chitosan brane formed on the substrate is cross-linked en supramolecular interaction, for example, hydrog ing The estimated mean pore size for the membr neutral conditions (pH 6.0 and 8.2) is about 17 nm the membrane at pH 3.6, it is 55 nm The contra swelling of the chitosan membrane are reversib fore, it is possible to control the pore size of the m by simply adjusting the pH of the system accord separation requirement (34) Smart polymers are the basis for a new pr lation technique—affinity precipitation The pre applies a ligand coupled to a water-soluble polym as an affinity macroligand, which forms a complex target The phase separation of the complex, trig small changes in the environment, for example, perature, ionic strength, or addition of reagents, m polymer backbone insoluble; afterward, the targe can be recovered via elution or dissolution Chito has been successfully used as a macroligand fo precipitation of isolated wheat germ agglutinin fr tract and glycosides from cellulose preparation by in the pH of the media (35) A partially sulfonated poly(ether sulfone) mic hollow fiber membrane was coated with chitosan b static attraction After cross-linking by reacting with ethylene glycol diglycidyl ether (EGDGE) droxyl and amino glucose units of chitosan are th fied to bind recombinant protein A (rPrA) as an af and at 4.77–6.43 mg rPrA/mL fiber The immobil hollow fiber membrane serves as a support for affi aration of immunoglobulin (IgG) (36) grafting of monomers onto the matrix The graft polymeric support, for example, chitosan-poly(glycidyl methacrylate) (PGMA), is used for glucosidase to immobilize urease (37,38) Immobilized urease can be used in biomedical applications for the removing urea from blood in artificial kidneys, blood detoxification, or in the dialysate regeneration system of artificial kidneys In the food industry, it may be used to remove traces of urea from beverages and foods and in analytical applications as a urea sensor In addition to urease, other enzymes, for example, glutamate dehydrogenase, penicillin acylase, β-galactosidase, and glucosidase, have been immobilized via chitosan gels Immobilization improves the stability of the enzymes Extracellular Matrixes For Tissue Engineering Various synthetic and naturally derived hydrogels have recently been used as artificial extracellular matrices (ECMs) for cell immobilization, cell transplantation, and tissue engineering Native ECMs are complex chemically and physically cross-linked networks of proteins and glycosaminoglycans (GAGs) Artificial ECMs replace many functions of the native ECM, such as organizing cells into a 3-D architecture, providing mechanical integrity to new tissue, and providing a hydrated space for the diffusion of nutrients and metabolites to and from cells Chitosan is similar to GAG Therefore, it is promising for application as a biomaterial in addition to use as a controlled delivery matrix Chitosan is a basic polysaccharide, so it is possible to evaluate the percentage of amino functions, which remain charged at the pH of cell cultivation (7.2–7.4) These cationic charges have a definite influence on cell attachment by their possible interaction with negative charges located at the cell surface Chitosan materials give the best results in cell attachment and cell proliferation for chondrocytes and keratinocytes of young rabbits compared with its polyelectrolyte complex with glycosaminoglycans, such as chondroitin 4 and/or 6 sulfate and hyaluronic acid (39) Field Responsive Materials Chitosan-based gels consist of a positive charged network and a fluid (e.g., water) that fills the interstitial space of the network The gels exhibit a variety of unique field responsive behaviors, such as electromechanical phenomena −70% Time (s) Figure 9 The EMC behaviors of chitosan/PEG crossl different concentrations of ECH Electromechanochemical (EMC) behavior d contraction of polymers in an electric field The an electric field on polyelectrolyte hydrogels rel protonation of its alkaline amino groups and r tion of mobile counterions when chitosan/PEG fibers is cross-linked with epichlorohydrin (ECH taraldehyde (GA), respectively The EMC behavio in a 0.1% aqueous HCl solution in a 25-V dc ele is shown in Fig 9 The bending direction of the fiber specimen a critical concentration of the cross-linking agen the ECH concentration is more than 9.0 × 103 M concentration is larger than 5.64 × 104 M, the fi mens bend toward the cathode If the ECH or G tration is less than the critical values, they ben the anode The reason may be attributed to va the mobile ions within the network (40) Thin films of chitosan and chitosan doped w earth metal ions can be used as wave-guiding m They are transparent across the wavelength ran 3000 nm and exhibit low optical loss (less than 0 (41,42) Chitosan/acetic anhydride and acrylate/chitos gels have an excellent laser-damage threshold to 35 times higher than commercial poly(met acrylate) (PMMA) bulk materials, and their LDT as water content increases As we know, absorbed ergy can lead to rapid local heating of a laser “ho hydrogel can be considered a composite of statist tributed microchannels and/or fluctuating pores c the movements of polymer segments within the in the presence of water When a hydrogel is i the energy generated by laser light can be abso dispersed by the water and the polymer frame drogels have potential applications as new ma high-power laser-damage usage (43) BIBLIOGRAPHY 1 J.-K F Sub and H.W.K Matthew, Application o based polysaccharide biomaterials in cartilage t neering: A review Biomaterials 21; 2589–2598 (20 Biotechnol 23(2), 175–192 (1995) 6 M.M Amiji and V.R Patel, Chitosan-poly(ethylene oxide) semi-IPN as a pH-sensitive drug delivery system Polym Prepri 35(1), 403–404 (1994) 7 X Chen, W.J Li, W Zhong, Y.H Lu, and T.Y Yu, pH sensitivity and ion sensitivity of hydrogels based on complex-forming chitosan/silk fibroin interpenetrating polymer network J Appl Polym Sci 65(11), 2257–2262 (1997) 8 K.D Yao, Y.J Yin, M.X Xu, and Y.F Wang, Investigation of pH-sensitive drug delivery system of chitosan/gelatin 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Calvo, C Ramona-Lopez, J.L Vila-Jato, and M Novel hydrophilic chitosan-polyethylene oxide ticles as protein carriers J Appl Polym Sci 63(1 (1997) 26 P Calvo, C Ramona-Lopez, J.L Vila-Jato, and M Chitosan and chitosan/ethylene oxide-propylene o copolymer nanoparticles as novel carriers for protein cines Pharm Res 14(10), 1431–1436 (1997) 27 A.B Schmuch and M.E Krajicek, Mucoadhesive p platforms for peroral peptide delivery and absorptio sis and evaluation of different chitosan-EDTA con Controlled Release 50(1–3), 215–223 (1998) 28 J.I Morata and T Ouchi, Chitosan derivati a function carrying DNA High Polym., Jpn (1997) 29 J.G Jegsl and K.H Lee, Chitosan membranes c with sulfosuccinic acid for the pervaporatio ter/alcohol mixtures J Appl Polym Sci 71(4) (1999) 30 E.E Skorikova, R.I Kalyvzhnaya, G.A Vikho Galbraikh, S.L Kotova, E.F Ageev, and A.B Ze plexes of chitosan and poly(acrylic acid) Macrom Sci USSR, Ser A 38(1), 61–65 (1996) 31 E.F Ageev, S.L Kotova, E.E Skorikova, and A.B vaporation membranes based on polyelectrolyte of chitosan and poly(acrylic acid) Polym Sci US Transl.), Ser A 38(2), 323–329 (1996) 32 T Uragami, K Tsukamoto, K Invi, and T Mi vaporation characteristics of a benzoylchitosan for benzene-cyclohexane mixtures Macromol Ch 199(1), 49–54 (1998) 33 S.Y Nam and Y.M Lee, Pervaporation sepa methanol/methyl-t-butyl ether through chitosan membrane modified with surfactants J Membr S 63–71 (1999) 34 W Wang, and H.G Spencer, Formation and ch tion of chitosan formed-in-place ultrafiltration m J Appl Polym Sci 67(3), 513–519 (1998) 35 I.Y Galaev, M.N Gupta, and B Mattiasson, Use s mers for bioseparations CHEMTECH 26(12), 19–2 36 E Klein, E Eichholz, F Theimer, and D Yeagor modified sulfonated poly(ether sulfore) as a support separations J Membr Sci 95(2), 199–204 (1994) 37 M Chellapandisn, and M.R.V Kridhnsn, Chi (glycidyl methacrylate) copolymer for immobilizat ase Process Biochem 33(6), 595–600(1998) 38 A Gallifuoco, L.D Ercole, F Alfani, M Cant Spagna, and P.G Pifferi, On the use of chitosan-im and W.W Adams, Characterization of chitosan and rareearth-metal-ion doped chitosan films Macromol Chem Phys 198(4), 1561–1578 (1997) 43 H Jing, W Su, M Brant, M.E De Rosa, and T.G Bunning, Chitosan-based hydrogels: A new polymer-based system with excellent laser-damage threshold properties J Polym Sci., Part B 37(8), 769–778 (1999) II–VI semiconductors (24), and rubbers (25) How in the elastic region has been observed only for t ated alkali halides (4,26), and some piezoelectric (27) So far nondestructive luminescence intensit terials have been reported to be too weak and d repeat, and this has deferred any practical app the phenomenon For application of ML in develo materials, repetitive ML must occur with und intensity COATINGS CHAO-NAN XU National Institute of Advanced Industrial Science and Technology (AIST) Tosu, Saga, Japan INTRODUCTION Many materials emit light during the application of a mechanical energy This phenomenon is usually referred to as mechanoluminescence (ML) or triboluminescence (1) The more historical term is “triboluminescence.” It stands for tribo-induced luminescence, and this was the term used for more than a century to refer to light emission induced by any type of mechanical energy (2) The term “mechanoluminescence” was not proposed until 1978 (3) The prefix “mechano” is correlated to the general mechanical way used for exciting luminescence, including concepts such as deformation, piezo, tribo, stress, cutting, grinding, rubbing, and fracto In recent years mechanoluminescence (ML) has become the preferred nomenclature (4) Although the transfer of mechanical stress into light radiation is very complex, successes in experimental applications suggest possible uses of the ML phenomena in stress sensors, mechanical displays, and various smart systems In general, ML can be divided into fractoluminescence (destructive ML) and deformation luminescence (nondestructive ML); these correspond to the luminescence induced by fracture and mechanical deformation of solid, respectively Roughly 50% of solid materials gives fractoluminescence by fracture (5): the well-known materials include sugar (6), molecular crystals (7,8), alkali halides (9,10), quartz (11), silica glass (12–14), phosphors (15), piezoelectric complex (16), metals (17), various minerals (18,19), and biomaterials (20) Recently, the fractoluminescence of rare-earth complexes was investigated Devices for ML Measurement Both mechanical and optical devices are being us sure ML The objective is to apply the measur anical energy to the ML sample, and then to d light induced by the mechanical energy The var niques already investigated include compression stretching, loading, piston impact, needle impact and cutting, laser, shaking, air-blast, scratching and milling, and tribo- and rubbing (4) Figur popular measurement devices for nondestruc these devices measure compression, tension, ben shearing Figure 1(a) shows a schematic diagr ML measurement device capable of measuring M stress relations simultaneously Stress is applie sample by a materials test machine The ML in measured by a photon-counting system that con photo multiplier tube (R464S, Hamamatsu Phot a photon counter (C5410-51, Hamamatsu Photo trolled by a computer The ML emission light is the photo multiplier through a quartz glass fibe spectrum is obtained with a photon multichanne system (PMA 100, Hamamatsu Photonics) Th ages are recorded with an image intensified charg device (ICCD) controlled by a computer system Hamamatsu Photonics Corp.) Simultaneously, and strain of the sample are measured by an in sor In addition to compressive test, the material chine shown in Fig 1(a) is able to apply tensile an stresses by exchanging the sample holder Figure 1(b) shows a schematic diagram of an surement device for applying friction (shear st same equipment as shown in Fig 1(a) is used to the ML intensity and spectrum The mechanic is applied by a friction rod under a load The fr material, as well as the load, can be changed for levels of mechanical stress applied to the test ma Spectrum (b) Strain amp Recorder hυ Sample V Speed controller Motor the test sample is rotated at a controlled speed, as shown in Fig 1(b), the friction rod draws a concentric circle on the test material The ML emission light induced by friction is guided to the PM through a quartz glass fiber that is 3-mm in diameter; the distance between the glass fiber and the friction tip is set at 40 mm The mechanical impact is applied by using a free-falling ball through a steel guide pipe The impact velocity is adjusted by the height of the falling ball, and the impact energy can also be adjusted by both the weight of the ball and the falling height NONDESTRUCTIVE ML FROM ALKALINE ALUMINATES DOPED WITH RARE-EARTH IONS As previously mentioned, development of materials with strong nondestructive ML intensity is an important goal in exploring applications of ML Recently, systematic materials research has resulted in producing a variety of materials that emit an intensive and repeatable ML during elastic deformation without destruction: among these are ZnS: Mn, MAl2 O4 :Re (M = alkaline metals, Re = rare-earth metals), and SrMgAl10 O17 :Eu (28–32) So far the most promising ML materials are the rare-earth ions doped alkaline aluminates and the transition metal ions doped zinc sulfide Remarkable upgrading in ML intensity has been achieved in the SrAl2 O4 doped with europium by controlling the lattice defects in the material Figure 1 Schematic diagram of ML me devices (a) using a materials test mach compressive test and (b) using a frictio chine for the friction (shear stress) test Preparation of Fine Particles of ML Materials and T Smart Coatings The luminescence powders are normally produ solid reaction process using a flux In the so tion process, the starting materials of ultrafine p SrCO3 , Al2 O3 , and Eu(NO3 )3 2H2 O, H3 BO3 (flux roughly mixed The mixture is calcined at 900◦ C f then sintered at 1300◦ C for 4 h in a reducing atm (H2 + N2 ) However, this process has the limita small particles cannot be obtained because of th of grains that occurs during the calcinations at hig ratures To address this problem, a modified sol-ge has been developed for preparing fine powders (33) In this modified sol-gel process, the starti rials of Sr(NO3 )2 , Al(O-i-C3 H7 )3 , and Eu(NO3 )3 dissolved in H2 O and thoroughly mixed with NH.3 sol solution is then dispersed by HCON(CH3 )2 , fo drying, calcining, and finally sintering in a red mosphere at 1300◦ C for 4 h In comparison wit powders synthesized by the solid reaction proc particles are obtained by the modified sol-gel proc mean size being about 1 µm; the particles obtain solid reaction process are about 15 µm The fin cles exhibit high ML, and as a result smart coati applied in uniform layers on the surfaces of the t jects by mixing it with binders and polymers For standard coating techniques such as spin coating a 50 0 0 5 10 15 X content in Sr1−x Al2O4−x (mol%) 20 Figure 2 Dependence of ML intensity on defect concentration of Sr coating could then be used to create uniform layers (films) of SAO-E/epoxy on the surfaces of plastics, rubbers, glass, ceramics, and metals The thicknesses of the resultant coating could be controlled from micrometer to millimeter Smart coatings made by mixing SAO-E fine powder with an optical epoxy can transfer mechanical energy into the photo energy of light emission, which in turn is capable of sensing the dynamic stress of substrate materials To obtain the ML intensity and stress distribution, the composite samples of the SAO-E/epoxy have been included in the ML measurement together with the coated samples and the ceramics of SAO-E ML Response of SAO-E to Various Stresses In upgrading the ML intensity of SAO-E for the smart coating application, the composition, the PH value, and the calcination conditions must be controlled Significant 2500 2000 120 1500 80 Io 1000 40 0 500 5 10 Time (sec) 0 15 Compresive load (N) Figure 3 (a) Typical time history of the luminescence intensity recorded during a compressive test for a SAO-E/epoxy composite with a dimension of 55 × 29 × 25 mm3 , (b) Diminishment in ML peak intensity during application of repetitive cycles of loading and its recoverability with UV irradiation of 365 nm Luminescence intensity (a.u.) 3000 Im 160 MLpeak intensity (Im−Io, a.u.) 100 The influences of the stress and strain rates on ted light intensity are measured using the same s but at different peak stresses, and then the s stress but at different strain rates Figure 3(a time history of a luminescent object recorded d application of a compressive stress with a const rate for a SAO-E/epoxy composite with a dim 55 × 29 × 25 mm3 As can be seen, over time increased load system results in linearly incre intensity That is, the ML intensity emitted i proportional to the magnitude of the applied st ure 3(b) shows the ML intensity diminished dur itive cycles of loading The ML intensity decre the repetitive cycles of stress, reaching a stabl about 20% of its initial strength The ML in SAO-E recovered completely after UV irradiation using a handy lamp Such recoverability dist SAO-E from other nondestructive ML materials for example, alkali halides which need γ -ray ir or very high energy irradiation to recover their i ity (34) The linear relationship between ML int and stress has been further demonstrated by th which each test had the same strain rate but a peak stress, as shown in Fig 4 The ML intensity linearly with the increase of strain rate Such a lation was also reported for γ -ray irradiated si tals of alkali halides during the elastic deforma It is evident that the ML of SAO-E is an elastic cence In a comparison test, the defect controll was found to give the most intense elasticolum among the materials examined to date (36) SAO-E was found also to exhibit ML dur tic deformation and fracture In the region 60 50 40 30 UV 20 UV 10 0 0 5 10 15 Repetitive load (cycles) 4.0 × 104 10 2.0 × 104 5 0 0 5 10 15 Mechanical stress (MPa) 0 Figure 4 Influence of applied stress on ML intensity (strain rate is 3.0 × 10−4 l/s) deformation, ML became intensified because of the stress that was concentrated in this region Upon further load increases the ML intensity exhibited a sharp rise as the SAO-E material began to crack, revealing the maximum value in ML intensity at fracture Similar results were obtained for ceramic samples of SAO-E The results for alkali halide crystals confirmed the presence of intense ML in plastic deformation and that it sharply increases during fracture of the crystals (4,5) Clearly, the linear relation between strain and stress is an important factor in the application of ML stress sensors Figure 5 shows the relationship between the strain rate and ML intensity The relationship is almost linear, as a higher strain rate produces greater ML intensity 90 80 ML intensity (a.u.) 70 60 50 40 30 20 10 0 0 5 10 Rate (mm/min) Figure 5 Influence of the strain rate on the ML intensity 0 0.1 Stress (MPa) Figure 6 Dependence of ML intensity on tensile stress E/epoxy composite sample with a dimension of 100 × 25 The ML intensity induced by compressive stress is also comparison Expressed mathematically, the ML intensity in the stress and strain rate is I − I0 = K σ (t)˙ (t), ε where σ is the stress and ε is the strain rate Equ ˙ indicates that the linear intensity of ML is consis the concept of an elastic (linear) region The dependence of ML on tensile stress is Fig 6 for a SAO-E/epoxy composite sample w surements of 100 × 25 × 5 mm3 For comparison intensity induced by compressive stress is also Note that the SAO-E exhibits the same ML whether or not the stress is compressive or tensile shows the dependence of ML on torsion measu shearing test machine for a SAO-E composite sam dimensions of φ10 mm × 110 mm Note that the M sity increases linearly with the increase of torsion the nondestructive ML of SAO-E can detect she and torsion changes without any volume change very different from the thermography technique detect stress based on the thermo-elastic effect wh is accompanied by a volume change As previously mentioned, SAO-E ceramics posites exhibite distinct ML behavior Smart coa SAO-E/epoxy can be applied to objects to sense c stress Figure 8 shows the dependence of ML int the stress for a plastic coated with an SAO-E/ep The results are similar to those of composites mics The ML intensity almost linearly increases ML 40 20 0 0 2 Torsion (kgcm) 4 6 Figure 7 Dependence of ML intensity on torsion measured by a shear test machine for a SAO-E composite sample with dimensions of φ10 mm × 110 mm increase of stress when the strain rate is kept constant Additionally, the ML intensity increases with the increase of the thickness in the ML layer A thick layer is generally not suitable for stress detection because it produces strain or stress in the ML layer that differs from that of the object beneath it As these results indicate, a uniform layer is necessary for an accurate display of the stress distribution of the object analyzer The broadband emission peaks at a wav about 520 nm, which is the same as the photolum (PL) spectrum measured by a fluorescence spec As shown in Fig 9, the PL and ML spectra fro are characterized by emissions that peak near 52 other emission bands are found in the ML spectr to 700 nm This implies that ML is emitted from emission center of Eu ions as PL; both are pro the transition of Eu2+ ions between 4f 7 and 4f 6 5d Emissions due to N2 discharge have not been which generally occur in destructive ML (fract cence) (41) These results confirm that the ML described here is produced by a nondestructive tion of SAO-E Moreover, the recovery of ML in UV irradiation suggests that the traps in SAOcan be filled by UV irradiation Measurements o effect of UV-activated SAO-E reveal traps of hole UV, and this is consistent with other reports (3 depths of these hole traps can be evaluated by th straaten method (42) This technique calls first fo luminescence glow curves to be measured at diffe of heating (β) to obtain the glow peak temperatur each heating rate β, and then for the depth of trap 100 250 PL 80 200 ML intensity (a.u.) ML intensity (a.u.) 30 µ 150 100 15 µ 50 0 60 DL 40 20 0 2 4 6 Stress (MPa) Figure 8 Dependence of ML intensity on the stress for a plastic block coated with SAO-E/epoxy layers with different thickness 0 400 500 600 Wavelength(nm) Figure 9 ML and photoluminescence (PL) spectra o Temperature difference (°C) Figure 22 Variation of the measured voltage with the temperature difference between hot and cold points of the epoxy-matrix composite junction comprised of bromine-intercalated P-100 and sodium-intercalated P-100 carbon fibers magnitudes of the absolute thermoelectric powers of the two partners because the electrons in the n-type partner, as well as the holes in the p-type partner, move away from the hot point toward the corresponding cold point As a result, the overall effect on the voltage difference between the two cold ends is additive By using junctions comprising strongly n-type and strongly p-type partners, a thermocouple sensitivity as high as +82 µV/◦ C was attained Semiconductors are known to exhibit much higher values of the Seebeck coefficient than metals, but the need to have thermocouples in the form of long wires makes metals the main materials for thermocouples Intercalated carbon fibers exhibit much higher values of the Seebeck coefficient than metals Yet, unlike semiconductors, their fiber form and fiber composite form make them convenient for practical use as thermocouples The thermocouple sensitivity of carbon-fiber epoxymatrix composite junctions is independent of the extent of curing and is the same for unidirectional and crossply junctions This is consistent with the fact that the thermocouple effect hinges on the difference in the bulk properties of the two partners and is not an interfacial phenomenon This behavior means that the interlaminar interfaces in a fibrous composite serve as thermocouple junctions in the same way, irrespective of the layup configuration of the dissimilar fibers in the laminate Because a structural composite typically has fibers in multiple directions, this behavior facilitates using a structural composite as a thermocouple array It is important to note that the thermocouple junctions do not require any bonding agent other than the epoxy, which serves as the matrix of the composite and is not an electrical contact medium (because it is not conductive) Despite the presence of the epoxy matrix in the junction area, direct contact occurs between a fraction of the fibers of a lamina and a fraction of the fibers of the other lamina, thus resulting in a conduction path in the direction perpendicular to the junction winding angle optimization (144) CONCLUSION Intrinsically smart structural composites for st ing, damage sensing, temperature sensing, control, and vibration reduction are attractive structures They include cement-matrix and matrix composites, particularly cement-matrix c that contain short carbon fibers and polymer-ma posites that contain continuous carbon fibers trical 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Polym Co 135–140 (2001) COMPOSITES, SURVEY K.H SEARLES Oregon Graduate Institute of Science and Technology Beaverton, OR INTRODUCTION The definition of a composite material, loosely translated, could point to any material having more than a single phase such that the multiple phases act in a synergistic manner to provide a behavior or set of behaviors more desirable than exhibited singularly By this definition, a significant amount of creative space is available for scientists and engineers alike to develop exciting new materials, structures, processes, and applications to meet the demands dictated by the evolution of new technologies As this evolution progresses, the traditional distinction made between the five classes of materials (i.e., metals, ceramics, polymers, elastomers, and glasses) tends to fade It is here where many new possibilities arise to intelligently process and “smear” these classes together to produce new, smarter composite materials and engineering structures that “think” and respond to stimuli The paradigm for this is by way of the tailorable structure-performance relationships for composites based on interactions between constituent materials, which may encompass any of the five material classes ENGINEERING MATERIALS The number of materials available to scientists and engineers for translating ideas into usable products and markets is vast, if not almost overwhelming Current estimations give a range somewhere on the order of between 40,000 and 80,000 engineering materials at our disposal (1) As new materials are developed having novel and exploitable characteristics (properties and responses), the number of choices further expands The appropriate choice(s) of material(s) for design can be just as overwhelming as the number of materials available How does one appropriately choose from the vast menu? Through proper rationalizations and design considerations, the search and selection processes can be drastically minimized Many of ternal two-dimensional and three-dimensional s such as interleaves, cells, and honeycombs co determine, to an extent, the external shape In terials, presence or absence of these structures fluences material behavior (response), ultimatel back to the desired function or set of functions trated by Fig 1, the interactions between functi rial, process, and shape are bidirectional At the these interactions, an iterative design process ut processing, materials, and design optimization sc forming an integrated system to arrive at a fina and market Materials Evolution Prior to 5000 BC, the engineering materials tha the peak of technology were natural composites, p ceramics, and glasses Little metal was consumed in the form of naturally occurring gold and silver 5000 BC and AD 500 the increased consumption bronze, and iron served to mark respective tech ages However, wood, skins, natural fibers, rub and pottery were still the primary sources for usage A perspective given by Ashby (1986) illust relative importance and evolution of advanced for mechanical and civil engineering purposes (1 It is evident from Fig 2 that engineering stee gain prominence until 1850, and from then to 1900s, steels and their alloys had been the preem terial sources for design Nonmetal materials we thought of for use as structural materials, perhap exceptions being refractories and Portland ceme so, their consumption compared to that of total consumption was small Within the last 20 to though, newly developed materials belonging to classes (polymers and elastomers, ceramics, co have surfaced due to their increasing technologic tance The growth rate of many of these mater par with (if not exceeding) the growth rate enjoye during the height of the Industrial Revolution Demand for New Materials Several factors must be taken into account rega choice of material substitution when designing considerable driving force behind this is weight However, part functionality must be critically re weight reduction is obtained through the substit lightweight material for a conventional one For Process Figure 1 Bidirectional interactions between function, material, shape and process (1) in a study of materials for ground transportation discussed by Compton and Gjostein (2,3), substitution of a hypothetical aluminum part of the same volume for a cast iron part would result in a 63% weight savings If equal load sharing were maintained, then weight savings dropped from 63% to 56% A similar comparison made between mild steel and high-strength steel revealed a weight savings of 18% where structural strength was the main concern, though mild steel is really no lighter than high strength steel by volume 10000 BC 5000 BC Gold 0 1000 1500 1800 Copper Bronze Iron For high-strength steel replacements using a or fiber-reinforced plastics (FRPs), savings in much smaller when equivalent tensile, compre bending stiffnesses are necessary Additional savings, another technological aspect of functio equally important pertains to the demand of elev perature environments In particular to the co aircraft, military aircraft, and aerospace secto temperatures from friction with air have risen 1900 1940 1960 Relative importance Glassy metals Al-lithium alloys Dual phase steels Microalloyed steels New super alloys Steels Elastomers Alloy Steels Glues Paper Pottery Glass Ceramics Glasses 10000 BC 5000 BC 0 2010 Development slow: mostly quality control and processing Light Alloys Super alloys Rubber Composites Rubber Flint 2000 Metals Cast iron Straw-brick 1990 Metals Polymers Wood Skins Fibers 1980 Polymers Elastomers High temperature Titanium polymers Zirconium Alloys High modulus polymers etc Composites Bakelite Ceramic composite Polyesters Metal-matrix Nylon Cement Composites Epoxies PE PMMA Acrylics Kevlar-FRP Ceramics Refractories CFRP PC PS PP GFRP Glasses Portland Tough engineering Fused Cermets PyroCement Ceramics (Al2O3, Si3N4, PSZ etc) Silica Ceramics 1000 1500 1800 1900 1940 Date 1960 1980 1990 2000 Figure 2 Relative importance and evolution of civil and mechanical engineering materials (1) 2010 rials to corrosion attack, and their ductility exposes them to failure by fatigue Ceramics and some glasses have relatively high elastic moduli, but unlike bulk and alloyed metallics, they are very brittle The strengths of structural ceramics and glasses are statistically dependent on volume, consequently these materials are termed “net shape” meaning little tolerance is available for changes in geometry without manufacturing an entirely new component They do function very well in hostile environments where temperatures, wear, and corrosion are excessive, but brittle behavior tends to foster catastrophic failure from concentrated stresses Elastomeric and polymeric materials have elastic moduli 40 to 50 times lower than metals, but they are as strong, if not stronger than metals They usually exhibit much higher strain to failure and properties that are much more dependent on temperature with a useful limit of 200◦ C (390◦ F) Some thermosets and thermoplastics can extend the limit by an additional 100◦ C (200◦ F), but this is typically accompanied by increased brittle behavior There are added benefits associated with processing and designing with polymers in that additional surface finishing is usually not necessary, resistance to corrosion is favorable, more complicated shapes are easy to form, and large deflections foster component designs that are flexible yet geometrically stable Contrary to the other material classes, composites tend to exhibit microscopic and macroscopic heterogeneity and behave anisotropically That is, composite mechanical properties vary from point to point due to the intrinsic properties of the reinforcements and their orientations From the standpoint of design, predicting responses to external mechanical and thermal loads becomes more complicated With the exception of mainly thermosetting polymer or nonpolymer matrices specifically tailored for elevated temperature environments, composites are also of limited use structurally above 250◦ C (480◦ F) Given this, composites do avoid some of the drawbacks associated with the other material classes while necessarily combining the attractive properties By incorporating various materials from principally diverse classes into the synthetically derived composite materials, several essential characteristics may be drawn upon: the corrosion resistance of polymers, the high strength and ductility, the light weight and lower cost of fabrication, the high temperature performance of ceramics and the thermal-electrical conductivity of metals (3) As shown by Fig 3, there is merit in grouping materials that show commonalities in properties and process Elastomer Glass Figure 3 Commonality of composite material proce properties to the principal classes of engineering mate COMPOSITE MATERIALS What are composite materials? If we adopt the ity that a composite material is any material con some combination of two or more phases, then alm material in the universe that exists naturally or ically may be referred to as a composite This g leads to a very broad classification of materials composites For example, wood can be classified posite, since its honeycomb microstructure cons rays of fibers (cellulose) encompassed and held by a polymer matrix (lignin) as shown in Fig macroscale, numerous combinations of the 60 n 30 imported species of wood are possible for tailo formance to suit a particular need (4) In many c macroscale wood composites serve as excellent b neering materials (plywood, particle board, etc.) Other, less-tailorable, natural composite mate biomaterials might also include bamboo, bone, cular tissues Bamboo, referred to as “nature’s o glass,” has an aligned fibrillar structure and e “broomed” fracture morphology similar to a fai fiber composite (5) The microstructure of muscul is such that flexibility occurs along with a high strength Fast- or slow-twitch fibers (collagen) ar in the general directions of loading along the bone surrounded by a continuous matrix having lower thus allowing neighboring fibers to slide past one Bulk engineering materials such as metal a plastics are traditionally not thought of as compos rials However, on the microscale, these materi well within the bounds given by the previous g Consider a heavily utilized bulk engineering mat plain carbon, hypoeutectoid steel (up to 0.8% ca though the separable phases require some for croscopy to resolve, they exist as fine dispers IC layer ric arrangement of laminae) At the macroscale inated composite and structural engineering or tural components are often synonymous What a review of composite principles and behaviors i the material interactions occurring at each of the beginning with the microscale interactions MICROSCALE BEHAVIOR Figure 4 Lamellar composite structure of wood cell composed of the primary wall cellulose fibers, a linear polymer in an irregular network, and the 3-layer secondary wall: S 1 —crisscross network, S 2 —spiral-type network, S 3 —irregular network The matrix is formed from the lignin and hemicellulose deposits act synergistically with temperature to control structural behavior under equilibrium conditions Upon cooling to below 800◦ C (1472◦ F), pearlite forms, which is a lamellar composite of the polyphases α+ carbide The layers of soft ductile alpha iron are interleaved with the layers of hard brittle iron carbide (cementite or Fe3 C), yielding a tough, strong steel Similarly, an engineering plastic like acrylonitrile-butadiene-styrene (ABS) is a prominent copolymer with rubber particles finely dispersed within the styrene-acrylonitrile (SAN) phase and, thus, is a microscale composite material The acrylonitrile provides surface hardness and heat resistance, while styrene contributes to strength and butadiene improves toughness and resistance to impact A succinct definition in the classical sense of what composite materials are is a prerequisite to understanding the structure-performance relationships that are possible and benefiting from the ability to tailor material performance through processing to suit end-use requirements Essentially, particulate and filamentary, namely fibrous, composite materials may be viewed as selective arrangements of hybrid materials of which their performance in structural and nonstructural applications is controlled through calculated variations of internal compositions and architectures More adequately defined, acceptable composites possess the following characteristics (5): r Consist of two or more physically distinct and mechanically separable phases r Can be dispersed or mixed in a controlled manner to achieve a desired behavior Fibers, Fillers, and Matrix Composites are nominally a synergistic combinat phases, the reinforcing phase and the matrix ph likely combinations of polymer matrix and poly epoxy-Kevlar), polymer matrix and ceramic (e glass), ceramic matrix and ceramic (e.g., Al2 O metal matrix and ceramic (e.g., WC–Co) The ma is a continuous medium wherein the reinforcing uniformly dispersed The majority of reinforcem in the form of either continuous aligned fibers o random fibers and dispersions or particles Othe quently used reinforcing materials include rib flakes Reinforcements vary in cross-sectional sh characteristic sizes ranging from circa 1.0 µm (3 mils) to 20.0 µm (0.78 mils) Since their diam small, the fibers, ribbons, or particulates cann structure singularly unless bounded by a matr geometrically stable Composites are often categorized according to of the reinforcements: dispersion-strengthened reinforced, and fiber-reinforced Dispersion-stre composites have small particles dispersed in which is the primary load-bearing constituent reinforced composites have larger particles inc within the matrix and the load is shared eq tween the particles and matrix Fiber-reinforce ites have either continuous (perhaps practically ous) or chopped fibers incorporated in a matri fibers are the primary load-bearing constituent Several materials are common to each form o ite reinforcement For fiber-reinforcements the selection of traditionally utilized materials tungsten, titanium, aluminum, boron, carbon from polyacylonitrile (PAN) precursors], arami (polyphenylene terepthalamide)], and glass (E, ECR grades) with compositions of SiO2 , Al2 O CaO, MgO, Na2 O, K2 O, BaO, and Ba2 O3 For ribb and particle reinforcements, the available se Kevlar 49 S-glass E-glass SiC Si3 N4 GPa (Msi) GPa (Msi) GPa (Msi) GPa (Msi) GPa (Msi) 125.0 (18.1) 72.0 (10.4) 84.0 (12.2) 380.0 (55.1) 380.0 (55.1) 3.2 (0.46) 6.0 (0.87) 4.6 (0.67) 2.8 (0.41) 1.0–10.0 (0.15–1.5) kg/m kg/m3 kg/m3 kg/m3 kg/m3 (lb/ft ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) 1450 2500 2550 2700 3800 Sources: Data after Courtney (p 224); Hull (5, p 14) materials includes glass, boron and graphite films, silicon carbide, and mica and aluminum of boride The list of usable materials for selection of a constituent matrix phase is vast, and composites are usually grouped into the material classes in which the constituent matrix belongs Many polymer matrix composites (PMCs) incorporate the following thermoplastic and thermoset polymers as constituent matrices: polyethylenes (PE), polyurethanes (PU), polyamides (PA-Nylon 6,6), polyether-etherketones (PEEK), polycarbonates (PC), epoxies, polyesters, and polyimides (PI), and bismaleimides (BMI) Ceramic matrix composites (CMCs) may incorporate the following advanced structural ceramics as constituent matrices: silica oxides (SiO2 ), alumina (Al2 O3 ), metallic carbides (W, Ta, Hf, Zr, Mo, V, Ti, Cr), nonmetallic carbides (B, Si), borides (Ta, V, Cr, W, Ti, C), metallic nitrides (Zr, Y, Mo, Ti, Cr), and nonmetallic nitrides (Al, B, Si) For metal matrix composites (MMCs), materials incorporated as constituent matrices include titanium, copper, aluminum, and nickel Specific material properties for some of the most reinforcing and matrix constituents are given in and 2, respectively Fiber-reinforced PMC’s are achieving widespre tance, particularly in the aerospace industry Co based on polymer epoxy matrices currently ac about 4% by weight of commercial aircraft and a of military aircraft with forecasts spanning the ne of 65% by weight (6) These composites posses specific strength, stiffness, and very large endu fatigue The room temperature specific strengths PMCs are higher than aluminum, titanium al some super alloys The upper service temperatur materials is limited to approximately 150◦ C (3 many aerospace PMC applications such as aircr and engines, however, temperatures often exce (392◦ F) and could reach 427◦ C (800◦ F) The st epoxy-based PMCs would begin to diminish far be temperatures, hence there is a growing demand Table 2 Selected Material Properties for Several Materials Commonly Used as Constituent Matrix Phases in MM CMCs, and PMCs Matrix Material Al2 O3 BN SiC Si3 N4 Epoxy Polyester Nylon 66 Polycarbonate Polypropylene Polyethylene-HD Urethane Polyvinylchloride Polyetheretherketone ABS (Acrylonitrilebutadiene-styrene) Acrylic Polyimide PMR-15 Units Tensile Modulus (E11 ) Tensile Strength (σ11 ) GPa (Msi) GPa (Msi) GPa (Msi) GPa (Msi) MPa (ksi) MPa (ksi) MPa (ksi) MPa (ksi) MPa (ksi) MPa (ksi) MPa (ksi) MPa (ksi) MPa (ksi) MPa (ksi) 470.0 (68.2) 90.0 (13.05) 380.0 (55.1) 380.0 (55.1) 4500.0 (652.7) 3250.0 (471.4) 2100.0 (304.6) 2300.0 (333.6) 1200.0 (174.0) 827.4 (120.0) — 2757.9 (400.0) 3650.0 (529.4) 2068.4 (300.0) 2.0 (0.29) 1.4 (0.20) 2.8 (0.41) 1.0–10.0 (0.15–1.5) 35.0–100.0 (5.1–14.5) 40.0–90.0 (5.8–13.1) 60.0–75.0 (8.7–10.9) 45.0–70.0 (6.5–10.2) 25.0–38.0 (3.6–5.5) 27.6 (4.0) 34.5 (5.0) 41.4 (6.0) 92.0 (13.3) 27.6–48.3 (4.0–7.0) MPa (ksi) MPa (ksi) 2895.8 (420.0) 4000.0 (580.2) 55.2 (8.0) 39.0 (5.7) Sources: Data after Flinn and Trojan, (4, pp 560–573); Hull (5, pp 29–33); NASA LeRC; DuPont AMS Units kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 De (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) (lb/ft3 ) 396 190 270 380 125 135 114 113 90 95 120 140 132 106 kg/m3 (lb/ft3 ) kg/m3 (lb/ft3 ) 119 135 faces for supersonics Materials used in these applications must possess a good balance of mechanical properties over a wide temperature range, withstand thermal shock and experience a low percentage of weight loss (thermal-oxidative stability, TOS) even after long-term exposure (60,000 h) at or near upper service temperatures To maintain sufficient mechanical properties, the polymer glass transition temperature (Tg ) must be at least 25◦ C (77◦ F) higher than its intended use temperature Currently, the PMR compositions are limited for use in applications operating in the 200 to 300◦ C (392 to 572◦ F) temperature range (7) γ = where A is a constant which scales the attractiv tions, n is the surface molecular density on eith the interface, and r0 is the equilibrium distance the mating surfaces near intimate contact For fib interactions, it is also useful to consider the re between the stiffness of the material and its su ergy in terms of the isothermal Young’s modulus E= Fiber-Matrix Interface In any composite system where one phase is completely dispersed within another phase, continuity between phases is extremely important When a continuum does not exist, the benefits associated with the combination of phases may not be fully realized In fiber-reinforced and many particulatereinforced systems, the matrix acts as a “conduit” for transporting load and sharing stresses equally within the reinforcing phase Whether or not an equivalent distribution occurs or concentrations arise becomes highly dependent on the structure and properties of the interphase, namely the fiber-matrix interface (particulate is referred to as a fiber from this point on) The interface acts as the medium by which stresses in the matrix are transported to the fiber, or alternatively, the significant differences between the elastic properties of the fibers and the matrix are communicated through the interface (5) How efficiently this communication occurs is governed primarily by the degree of bonding that takes place between the fibers and the matrix Fiber pullout may result if the interfacial bonding is poor and if the interfacial bonding is too good, then energy absorption subsequent to the debonding process will be insufficient Ideally, debonding of the fiber-matrix interface should follow fracturing of the matrix (8) The nature and efficiency of bonding between a fiber and matrix will be dependent on the molecular conformation and chemistry of both constituents Even though the interface is specific to each fiber-matrix system, insight can be given toward the origins of bond strength by way of the principal mechanisms governing adhesion, chiefly wetting, interdiffusion, electrostatics, chemical bonding and mechanical interlocking Contact and Wetting Central to the study and application of adhesion wetting mechanisms, the Dupre equation πn2 A , 2 32r0 32γ r0 The isothermal Young’s modulus E of a mat also be expressed by the following relationship: E = r0 ∂φ L−J (r) ∂r T,r =r0 , where φ L−J represents the potential energy acc Lennard-Jones proposed interactions between a molecules While the work of adhesion WA can be as a thermodynamic parameter that describes necessary for two mutual surfaces to remain in contact, the work of cohesion WCOH may be re the work required to create two new surfaces of material as in the case of crack origination If a m material is broken such that the separated mat completely nonenergy dissipating nor absorbing brittle), then the work of cohesion is simply 2γ One might now discern that cohesion is adhe consideration for the respective materials in co their interfacial energy existing prior to a separ extending the concept of surface energies furthe ting adhesion mechanism can be understood in the Dupre equation and the Young equation T equation relates the physical occurrence of a li on a solid surface It states that γ LV cos θ = γSV − γSL , where γij are the appropriate interfacial tension the liquid L, solid S, and vapor V and θ is the con as shown in Fig 5 The solid–vapor interfacial en the true surface-free energy of the solid, but it γSV = γS − πe Here γS is the true surface free energy and πe is referred to as the equilibrium spreading pressure, a measure of released energy through adsorption of the vapor onto the solid surface If combined, the Dupre equation with the Young equation yield a simple expression (Dupre-Young) that relates a thermodynamic parameter to two readily determined quantities: WA = γ LV (1 + cos θ) Knowing the liquid–vapor interfacial tension and the contact angle, the work of adhesion is easily calculated In considering the wetting of a fiber by a matrix, only liquids with γ LV less than the critical surface tension of wetting will spontaneously spread on the fiber For example, epoxy has a surface energy of 042 J m−2 (0.100 × 10−1 cal in−2 ), glass a surface energy of 0.560 J m−2 (1.337 × 10−1 cal in−2 ), and graphite a surface energy of 0.077 J m−2 (0.184 × 10−1 cal in−2 ) Although epoxy will wet both glass and graphite, the glass will wet more readily due to a much lower contact angle between the epoxy and glass When fibers are considered for use with matrices having a greater surface energy, coatings and sizings are often introduced via polymers with lower energies to promote better wetting and adhesion characteristics Interdiffusion The mechanism of adhesion by interdiffusion is thought to occur when the two surfaces that are brought into intimate contact are at least partially soluble in one another If we adopt the philosophy that “like dissolves like,” then only a limited number of adhesive examples are applicable to a mechanism of adhesion by interdiffusion, unless coatings are used that are very similar as in some fiber-reinforced composite applications In any case, diffusive bonding implies that an interphase forms between two materials that is a mixture of both materials as shown in Fig 6 The extent to which the interphase is equally apportioned and continuous between the two materials depends on the solubility parameter criterion, defined as δ= ECOH , V where V is the molar volume and the cohesive energy ECOH , in this case, is the energy required to separate all of the atoms or molecules in a material an infinite dis In terms of the enthalpy of vaporization Hvap constant R, and the absolute temperature T, the energy is given by the following relationship: ECOH = Hvap − RT This equation implies that the extent of energy sion is indicated by the energy required for vap Spontaneous formation of a solution is dependent magnitude and sign of the Gibbs free energy of m Gmix = Hmix − T S mix Polymer materials are limited to a few states because of their high molecular weight Therefor tropy change S is small and the enthalpy change ther zero or positive, meaning that dissolution an between polymers is not likely Complete interpha tion requires a larger entropy change leading to a Gibbs free energy of mixing Perfect adhesion by requires two materials that are completely solub another, giving the most negative value for chan thalpy of mixing which would be zero In polyme als, autohesion (interdiffusion) may be assisted b dition of plasticising agents or solvents ultimately the degree of molecular entanglement and bond Electrostatic Interactions Adhesion by ele interaction appears to be an unlikely contribu rect bond strength in composite materials How mechanism of adhesion provides a reasonable jus for acid–base interactions and the use of couplin In theory, it can be stated that surfaces that are electropositive in character, whereas surface acidic are electronegative in character Accordin should be some strength of attraction between t surfaces, with the greatest adhesion by electrosta action occurring when one material is the most ba the other material is the most acidic Coupling ag as silane can be introduced to couple an organic p an inorganic phase These agents possess chem tions that are twofold: they are reactive with both ganic and organic phases Fiberglass utilizes such agents to protect the glass fiber–matrix interfac attack by moisture and subsequent debonding layer is formed between the glass fiber and st formed between compatible chemical groups on the fiber surface and matrix, the bond strength will depend highly on the number of bonds as well as the types The energy required to separate the two materials at their interface will need to be greater than the covalent bond energy The bond energy is attributable not only to the activity of the chemical groups but also to the surface area, since a larger area can provide a greater number of chemical bond sites The silane coupling agents R–SiX3 added to the surface of glass fibers ensure good wetting of the fibers in the presence of a hydrated water layer, which tends to greatly reduce the glass fiber surface energy These agents also provide a strong chemical link between the glass surface oxides and the polymer matrix, leading to a strong, waterresistant bond Strong bonds between carbon fibers and matrices occur due to the highly reactive surface structure and large surface area (a large surface microroughness) of carbon fibers If the carbon fiber surface is treated in thermal and acidic environments, ranges of functional groups can be produced that bond directly with unsaturated matrix groups and unsaturated thermoplastic matrix groups Mechanical Interlocking Adhesion by the mechanism of mechanical interlocking is possible, provided that a sufficient surface roughness exists in conjunction with a high degree of wetting The higher the surface wetting, the more likely the matrix is to conform with the existing fiber surface topography The lower the surface wetting, the more likely the matrix is to fill interstices lending a smooth surface and little adhesion by mechanical interlocking The existence of sharply transitioning topography does two things to promote adhesive strength between materials: roughness increases the effective area of contact, and developing cracks are forced to propagate along a tortuous path Whether or not the matrix fills the pores and smooths topography can be described by Poiseuille’s law, which states that x dx dt = r2 P , 8η where x is the distance of pore penetration by the matrix, t is time, r is the pore radius, P is the capillary pressure, and η is the polymer viscosity The capillary pressure can also be related to the liquid–vapor interfacial energy and the wetting contact angle in terms of the pore radius by and stiffnesses of the constituent materials At t and mesoscales, the transfer of load, and elastic a behaviors of composites involve many factors b constituent material properties Generally spea primary factors that influence lamina behavior a r The properties of the constituent reinforce matrix materials r The properties of the interphase bonding stituent materials (previous section) r The volume fractions of the constituent ma r The geometry of packing (aligned, random etc.) r The cross-sectional shapes and aspect rat reinforcements A single composite lamina may have mesos erties and exhibit behavior that are far removed properties and behavior of the constituents For the constituents can be isotropic while the lamin orthotropy of strength and modulus As a resul ite mechanical behavior is often categorized as ther fully isotropic or plane orthotropic with t anisotropy That is, the properties are conside hibit orthotropic behavior in the principal reinfor and to be equivalent in planes transverse to the ing plane Because of their complexity, they ar never considered to be fully anisotropic as 21 con required to completely describe fully anisotrop behavior The number of possible composite arc (classes), with textile classes defined to a large NASA Langley Research Center’s (LaRC) Advan posites Program (ACT), fall into one of the categ are presented in Fig 7 (11) When evaluating orthotropic composite beha useful to establish a coordinate system relative t sian or polar coordinate system and coincident inforcing direction This principal material coord tem (1–2–3) serves to establish the nine elastic that apply for orthotropic behavior, namely the nal tensile modulus, the shear modulus, and Po tio (E11 , G12 , ν 12 ) and the transverse tensile mod moduli, and Poisson’s ratios (E22 , E33 , G13 , G23 However, by assuming orthotropic symmetry, on duce the number of effective elastic constants re describe orthotropic behavior from nine to five in Fig 8 for a unidirectional (UD) composite lam Continuous matrix Particulates 2D Braids Discontinuous fibers Aligned Bias triaxial 3D Tubular cartesian Random 2-step 4-step multi-step Figure 7 Possible fiber and textile composite architectures (classes) for general and advanced structural high-performance use categorized, in large part, by NASA LaRC’s ACT program established in 1989 leads to the following relations: E22 = E33 , G23 = v12 = v13 , E22 (or E33 ) , 2(1 + v23 ) G12 = G13 Continuous Fiber Reinforcement The elastic behavior of a composite lamina having continuous fiber reinforcements is evaluated on the basis of the constituent material properties and their associated volume fractions In a continuous fiber, UD composite lamina (Fig 8), all fibers are assumed to be aligned in parallel and extend to the boundaries of the matrix For the purposes of theoretical analysis, the fibers are considered to be arranged in an ideal square or hexagonal array (analogous to the Kepler conjecture) giving a fiber volume fraction Vf according to Vf = 0.25π r R 2 : square, Vf = 0.29π r R 2 : hexagonal, where 2R is the center-to-center fiber spacing and 2r is the fiber diameter In practice, however, the fiber volume fraction is usually determined by ASTM standardized acid digestion techniques or image analysis using some form of microscopy On the premise of linear elasticity, when a load is applied parallel to the lamina fibers, the strain experienced by both the fibers and matrix is the same (isostrain condition) Since the fiber stress is σf = Ef ε1 and the matrix stress is σm = Em ε1 , it follows that the fiber stress in the fibers than in the matrix (Ef > Em ) If w that a total load Pc is applied and the bonding constituents is continuous, then Pc = Pf + Pm , w subscripts c, f, and m denote the composite, fiber trix, respectively Knowing that the strain in the c is the same as the strain in the fibers and matrix stresses are a function of the applied load and a tions, the isostrain condition can be explained by σc σf σm = (Vf ) + (Vm ) εc εf εm Upon substitution of the definitions for the fiber trix tensile moduli in this equation, the tradition mixtures” equation based on an isostrain loading is defined: E11 = E = Ef Vf + Em (Vm or (1 − Vf )) Loads applied to the composite lamina that ar dicular or transverse to the fiber direction follow derivation, but the loading is referred to as an condition (see Fig 9 for isostrain vs isostress) U isostress assumption, the stresses are assumed same in the lamina, fibers, and matrix However th become a function of the area fractions of fibers an to that of the total composite It follows that the condition can be explained by εc = σ2 σ2 (Vm ) + (Vf ) Em Ef 0.0 X1 (Fiber direction) E11 G12 ν12 E22 0.2 0.4 0.6 Fiber volume fraction 0.8 Figure 9 Schematic of isostrain versus isostress lo composite material, indicating that less fiber is require to isostrain assumption for yielding the same elastic mo isostress assumption approaches the isostrain assump ratio of elastic constituent moduli Ef /Em = 1 E22 G23 E11 ν23 Figure 8 Representation of the principal material coordinate system in a UD composite lamina exhibiting orthotropic symmetry and the effective elastic constants derived from the basic loading schemes Upon substitution of the definition for stress (Hooke’s law) in the preceding equation, a more traditional equation for the transverse tensile composite modulus is defined: E22 Ef Em = E1 = E f (1 − Vf ) + Em Vf Composite lamina elastic properties derived on the basis of the isostress loading condition provide only marginal agreement with actual results The isostress derivation does not take into account the effects of the Poisson contraction Therefore other mesomechanics approaches are necessary for evaluating lamina elastic constants from constituent materials that exhibit highly anisotropic elastic behavior According to more realistic assumptions that account for the Poisson contraction and nonuniform stress distributions due to strain magnification between neighboring fibers, additional solutions obtained from elasticity theory and finite element analysis have been offered for predicting transverse and longitudinal elastic properties One such set of solutions that are generally more applicable are the Halpin-Tsai equations (12) These equations can be written in the general form as: 1 + ξ ηVf ( r)c = , 1 − ψηVf ( r)m where ( r) is the elastic constant, ξ depends on th teristics of the reinforcing phase such as fiber as packing geometry, shape, and loading conditions a factor that accounts for the maximum packing The other factor η is given by η= r−1 , r+ξ ( r)f r= r ( )m When the factor ψ is properly evaluated for m packing fraction, the general form of the Halpin-T tions for UD lamina elastic properties can be gi panded notation as E11 = E = Ef Vf + Em (1 − Vf ), ν12 = ν + = νf Vf + νm (1 − Vf ), (1 + ξ ηVf ) M , = Mm (1 − ηVf ) η= (Mf /Mm ) − 1 , (Mf /Mm ) + ξ in which M is the elastic modulus of interest, or ν23 ; Mf the constituent reinforcing fiber modu νf ; and Mm the constituent matrix modulus Em The reinforcement factor ξ is determined empi curve-fitting the experimental results When ξ Halpin-Tsai formulas lead to the classical rule-of relation, and when ξ → 0, the Halpin-Tsai form to the inverse form of the rule-of-mixtures: ( r)c = Vf ( r)f + 1 − Vf ( r)m , 1 − Vf Vf 1 = r + ( r)c ( )f ( r)m For reinforcing fibers such as carbon or Kevlar, hibit anisotropic behavior but are considered tra isotropic, the following expressions from Ch 23 where the subscripts f, m, i j f and i jm(i, j = 1, 2) denote the elastic properties of the reinforcing fibers and matrix Composite cylinder assemblage (CCA) theory also provides simple, closed-form analytical solutions for the effective composite moduli and the fiber and matrix are also considered to be transversely isotropic (14–16) CCA yields close-bounded solutions for the transverse tensile and shear moduli and models the composite as an assemblage of long, circular fibers surrounded by concentric matrix shells The dependence of the effective elastic moduli on fiber volume fraction is presented in Fig 10 and 11 for two different composite systems, a E-glass-epoxy composite lamina and a carbon-epoxy lamina It is apparent from the results that packing densities and constituent material elastic behaviors can readily be used as parameters in the design of composite materials when tailoring the performance characteristics Textile Architectures Many of the woven, braided, and stitched fabric-reinforced (textile) composite laminae also E11 E22 ratios 25 E11/Em E22/Em G12/Gm 20 G12 ratio 7 X3 (Thickness direction) X2 (Transverse direction) 6 5 15 4 X1 (Fiber direction) 3 10 2 5 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Fiber volume fraction 0.8 0.9 1 0 Figure 10 Effect of fiber volume fraction (packing density) on the effective elastic modulus ratios for a E-glass-epoxy composite system (solution of Halpin-Tsai) Constituent properties: E11f = E22f = 72.0 GPa (10.4 Msi), G12f = 27.7 GPa (4.02 Msi), ν12f = 0.30, E11m = E22m = 3.5GPa (0.51Msi), G12m = 1.3 GPa(0.19Msi), ν12m = 0.35 5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 Fiber volume fraction Figure 11 Effect of fiber volume fraction (packin on the effective elastic modulus ratios for a car composite system (solution of Chamis) Constitu erties: E11f = 230.0GPa(33.4Msi), E22f = 40.0GPa(5.8M 24.0GPa(3.48 Msi), G23f = 14.3 GPa(2.07 Msi), ν12f = 0 E22m = 3.5 GPa(0.51 Msi), G12m = 1.3 GPa(0.19 Msi), ν1 fall under the umbrella of continuous fiber reinfor However, nearly all of the textile architecture aligned within the reinforcing plane These fiber tures provide reinforcement in two or three direct the disadvantage of having somewhat lower in-p nesses due to the textile geometries Textile re phases do afford better shaping capabilities ov surfaces (drape) and the possibility for throughreinforcement as with three-dimensional archite In contrast to aligned fiber, UD laminae, tex forcements consist of many filaments grouped to act as primary braiders, weavers, and fillers (w weft tows) Fiber bundling influences not only ing arrangement and density but also the cross shapes and aspect ratios Such variations in the combined with different textile architectures requ nate methods to evaluate the mechanical respon composites For example, consider the eight harn (8HS) weave architecture shown in Fig 12 The bu amentary tow cross-sectional shapes are biconve ular); that is, thickness variation exists Additio warp tows interlace at every eighth mutually o weft or fill tow (hence, 8HS) The periodic interla orthogonal tows permit shapability, but increas plane compliances Because of these factors and ot necessary to evaluate the effective composite prop the basis of transformed reduced stiffnesses and thickness homogenization schemes that smear properties into an effective continuum The complete evaluation of elastic and the tic behavior of woven and braided composite is involved and beyond the scope of this revie ever, the reader is encouraged to study referen 23 for thorough analytical and numerical treat the subjects of textile composite elastic and behaviors) Weft (fill) tows Warp tows Figure 12 Unit cell of an 8-harness satin (8HS) woven reinforcing architecture consisting of weaver (warp) tows interlaced with filler (weft) tows Maximum width of the filamentary bundles is given by a and the cross-sectional shape is lenticular Randomly Oriented Fiber Reinforcement Continuous Fibers As with laminae constructed using continuously aligned fibers, a lamina may also be made by impregnating a polymer matrix into continuous and discontinuous fiber mats having fibers at various orientations For the case of continuous fibers, lamina elastic properties can be considered isotropic in plane so long as a uniform probability exists over the entire range of fiber angles from −π /2 to + π/2 Neglecting fiber end effects, it is only necessary to determine the average composite longitudinal tensile and shear moduli Akasaka derived the following expressions for the averaged effective elastic constants of composites based on randomly oriented, continuous fibers (24): Ec = E11 + E22 + 2ν12 E22 1 − ν12 ν21 × Gc = νc = E11 + E22 − 2ν12 E22 + 4(1 − ν12 ν21 )G12 , 3(E11 + E22 ) + 2ν12 E22 + 4(1 − ν12 ν21 )G12 These sets of equations infer the possibility making composite laminae with fiber distributio a priori so that mechanical, thermal, and othe properties can meet the specified requirements design Discontinuous Fibers Behavior of composite made from randomly oriented, discontinuous fibers present a more complex problem in so far a ing mechanical response because of the existenc end effects The assumption of isostrain does n with continuous fiber systems because the strai from the fiber-end–matrix interface is not the sa transfer borne by continuous fibers along their le der an applied tensile force, the matrix displace to the fiber along the fiber–matrix interface resul terfacial shear stress The relative displacement the fiber midpoint and maximum at either end o mination The relationship between the fiber te shear stress is (d/dx )σx = (4τm /df ), and the varia distance along the fiber is shown for the case of e sile deformation in Fig 13 The load transfer from the matrix to the fib over a given length of fiber called the critical leng is just enough to allow for complete transfer Th length is such that complete transfer takes plac fiber breakage or matrix failure and is defined a E11 + E22 − 2ν12 E22 1 + G12 ; 8(1 − ν12 ν21 ) 2 Ec 2Gc − 1 A simpler approach that produces similar results can be used the above equations for random fiber orientation in two dimensions: 3 E11 + 8 1 Gc = E11 + 8 Ec = 5 E22 , 8 1 E22 , 4 where E11 and E22 are the longitudinal and transverse tensile moduli along and across the fibers in a UD composite having the same fiber volume fraction These two approaches are equivalent to assuming that there is an equal lc = df Ef εm 2τym , where the subscripts f, m, and ym denote the fibe and yield of matrix, respectively The critical as of fiber length-to-fiber diameter (lc /df ) increases posite stress and strain according to the previous If we define longer fibers to be l > lc and shorte be l < lc , then the shorter fiber, composite effect tudinal tensile, and shear elastic moduli become Ec = 1 lc Vf Ef 1 − 3 2l + (1 − Vf )Em , Gc = 1 lc Vf Ef 1 − 8 2l + (1 − Vf )Em , for the two-dimensional consideration For rand tations of longer fibers in two-dimensions, the ... ± 0.5) × 10 4 (4. 8 ± 0 .4) × 10 4 (5.6 ± 0.5) × 10 4 (3.2 ± 0.3) × 10 4 (1. 4 ± 0 .1) × 10 5 (1. 1 ± 0 .1) × 10 5 (1. 5 ± 0 .1) × 10 4 (8.3 ± 0.5) × 10 2 (9.7 ± 0.6) × 10 4 (1. 8 ± 0 .2) × 10 3 − 51. 0 ± 4. 8 −56.8... application of different stresses, includ ing, tension, compression, and impact ( 62) The M (c) O 17 1E? ?15 341 E? ?15 512 E? ?15 682 E? ?15 853 E? ?15 10 2 E? ? 14 11 9 E? ? 14 13 6 E? ? 14 15 4 E? ? 14 Figure 17 Stress... State Chem 11 4: 297 (19 95); 12 0: 2 04 (19 95) 12 C Zener, Phys Rev 82: 40 3 (19 51) 13 P.G de Gennes, Phys Rev 11 8: 14 1 (19 60) 14 P.W Anderson and H Hasegawa, Phys Rev 10 0: 675 (19 55) 15 A.J Millis,